Because each person has a differently shaped head and these articles are mass produced and must fit everyone, the edge of the hollow portion is usually made of or provided with a resiliently deformable material, such as soft plastic or resiliently compressible plastic foam. In order to provide a suitable fit for the user the soft edge is pressed resiliently against the portion of the head which it is required to fit, so that the elastic deformation of this edge provides the best possible fit on the surface of the head against which it is applied, and forms a leak-tight seal to isolate the hollow portion from the surroundings, in order to shield the ears or eyes from the surrounding medium, either to protect them or to isolate them from external noise. The pressure exerted is a function of the size of the irregularities to be absorbed by the seal and the degree of compressibility of the material forming the seal. Consequently certain parts of the head are subjected to comparatively high local pressures which make wearing of the device uncomfortable or even painful after a certain time. In addition, the fit may not be very good, even with a non-negligible pressure, for example in the case of protective goggles for motorcycling or skiing where the seal frequently does fit properly on the base of the wings of the nose, leading to air infiltration which may cause ophthalmia.
Use has already been made of low density, nitrogen blown, polyethylene heat-formable foams in the medical field, in particular in order to mold this foam around various parts of the body in order to provide splints or orthopaedic supports or even to fit a prosthesis to an injured limb. The foam is heated to its softening temperature which is approximately 140.degree. C. and is applied to the part of the body to which the foam is to be fitted, the foam being formed until it completely matches the desired shape.
A new application of this foam has been proposed, which consists in fitting an article of clothing to a part of the human body, as disclosed in European Patent Publication 0004829. In accordance with this publication, a plastics foam lining at least partly covers the internal face of a non-extensible casing and provides within this casing a receptacle whose dimensions are smaller than the corresponding dimensions of the part of the body to be fitted, so that the introduction of this part of the body into the cavity and the heating of the foam to its thermo-forming temperature enables the substantial stresses exerted on the part of the body to be eliminated and enables a cavity having dimensions which fit the part of the body to be provided. This technique is used to fit ski boots or helmets.